Optical inclination sensor

ABSTRACT

An optical inclination sensor is provided having at least one reflective surface and at least two separate optical fibers having ends spaced from a reflective surface. As the reflective surface tilts with respect to a pre-determined reference position the gap lengths between the fiber ends and the reflective surface change and the differences in these gap lengths is used to calculate an angle of inclination with respect to a reference position. The optical inclination sensor can include at least one mass attached to a housing and moveable with respect to the housing as the mass and housing are rotated about one or more axes. Optical strain sensors are disposed a various locations between the mass and housing so that as the mass moves with respect to the housing, each one of the optical strain sensors are placed in compression or tension. The housing can be a generally u-shaped housing having two arms and a base section with the mass disposed within the housing. Alternatively, the housing includes a first beam, and the mass is a second beam arranged generally orthogonal to the first beam and pivotally attached thereto. The optical strain sensors are disposed between the first beam and the second beam. The optical strain sensors are placed in tension or compression as the second beam pivots with respect to the first beam.

FIELD OF THE INVENTION

This invention generally relates to sensors for measuring angles ofinclination. More particularly, the present invention relates toinclination sensors utilizing optical interferometers.

BACKGROUND OF THE INVENTION

Tilt meters or tilt sensors are used in the geophysical sciences andother applications to measure tilt or inclination of the ground or ofstructures. These measurements can be taken at the surface or,particularly in the case of geophysical applications, below the surfaceof the Earth in, for example, gas or oil wells. In these applications,the tilt meters are used to provide information about the general shapeof a well or of sudden turns in the well. For example, oil wells canextend 10,000 or 30,000 feet below the surface of the Earth. At thesedepths, the wells can develop helical or corkscrew bores. These boresneed to be monitored and tracked during the drilling process. Inaddition, highly sensitive tilt sensors are used in oil and gas wells todetect subtle changes in the Earth's structure, to measure subsidence,shifting or the quality or effects of nearby fracturing operations.

In general, tilt sensors are arranged to provide an indication of whenthe sensor changes orientation with respect to a predeterminedreference, for example tilting with respect to horizontal, vertical orwith respect to the direction of the Earth's gravitational pull. Thesesensors can provide a simple indication of tilt or can be calibrated toprovide a measurement of the degree of tilt. Conventional tilt sensorsare electronic devices that utilize a pendulum type sensor or a magneticresistance element. Other types of tilt sensors include potentiometertype, servo type, bubble type, capacitance type and mercury type.

Other types of tilt sensors have combined optical elements withelectronic elements. For example, U.S. Pat. No. 4,726,239 is directed toa soil analyzer and penetrator that includes a cone connected to thelower end of a hollow tube for measuring various ground parameters. Acone angle measuring device, which is an optical tilt meter, includes alight source, a photo sensor and a concave lens positioned between thelight source and photo sensor. A steel ball rides on the concave surfaceof the lens. Therefore, the light from the source is limited in reachingthe sensor depending upon the position of the ball. As the sensor istilted, the ball rolls on the concave surface of the lens allowing morelight to pass. Therefore, the output from the photo sensor isproportional to the angle of tilt of the sensor and thus of the cone.

Similarly, U.S. Pat. No. 5,134,283 is directed to an optical detectionapparatus whereby the tilting state, such as rolling or pitching state,of an object, may be detected by a simplified optical system. Theoptical detection apparatus includes a light emitting element providedat distal end of a moveable member for radiating light downwards. Acondenser lens is mounted below the moveable member, and a photo sensordevice, for example a photodiode device, is mounted below the condenserlens. A signal processing circuit determines the position on the photosensor device of a light spot from the condenser lens.

One system utilizes optical fibers in combination with conventionalelectronic optical detectors. U.S. Pat. No. 4,812,654 is directed to atwo-axis quartz fiber passive tilt meter utilizing a quartz fibersuspended for emitting radiation from a distal end thereof and apendulous mass suspended from the quartz fiber to improve the pendulousresponse. The infrared radiation transmitted and emitted by the quartzfiber is directed by a lens system to strike an axially displacedposition on a detector producing DC signals representative of theintensity of light falling on the respective detector quadrants.

The combined systems still utilize electronic components that limits howsmall or compact the sensor can be. In addition, electronic sensors canbe influenced by electromagnetic effects, temperature and signalattenuation, especially in deep well applications.

Therefore, a need exists for a tilt meter that is compact and suited forsubsurface measurements. The tilt meter would obviate the need forelectronic components located distally at the point of measurement,thereby eliminating electromagnetic interference.

SUMMARY OF THE INVENTION

The present invention is directed to an optical inclination sensorhaving at least one reflective surface and at least two separate opticalfibers having ends that are spaced from the reflective surface. As thereflective surface tilts with respect to a pre-determined referenceposition or around a pre-determined axis, the gap lengths between thefiber ends and the reflective surface change and the differences inthese gap lengths are used to calculate an angle of inclination withrespect to the reference position or axis. The optical inclinationsensor provides a resolution in gap length as low as about 0.1 nm. Ingeneral, the optical inclination sensor operates as an interferometer,either intrinsic or extrinsic, to measure these differences in gaplength.

The optical fibers are fixedly attached to a housing, so as to rotate ortilt with the housing. The housing can be arranged as a generallyu-shaped capillary tube, and the reflective surface is the surface of aliquid, for example mercury, disposed in the capillary tube. Inaddition, the optical inclination sensor can include more than twooptical fibers, and the housing can include a manifold assemblycontaining a sufficient number of ports to hold each optical fiber.

The present invention is also directed to an optical inclination sensorthat includes at least one mass attached to a housing or frame andmoveable with respect to the housing as the mass and housing are rotatedabout one or more axes. Optical strain sensors are disposed at variouslocations between the mass and housing so that as the mass moves withrespect to the housing, each one of the optical strain sensors areplaced in either compression or tension. The compression and tension ofthe optical strain sensors are used to calculate an angle of inclinationof the housing and mass with respect to each axis.

The optical strain sensors are arranged as pairs and positioned so thatone optical sensor is in compression when the other optical sensor is intension as the mass and housing rotate about a given axis. The housingcan be a generally u-shaped housing having two arms and a base section.Each optical strain sensor is disposed between the mass and one of thetwo arms, and the mass is moveably attached to the base section.Suitable optical strain sensors include (i) extrinsic Fabry Perotinterferometers having a gap comprising a predetermined length and eachsensor is attached between the mass and the housing so that movement ofthe mass with respect to the housing changes the length of each gap and(ii) intrinsic optical sensors such as a fiber Bragg grating, amongothers.

In another embodiment, the housing is arranged as a first beam, and themass is a second beam aligned generally orthogonal to the first beam andpivotally coupled thereto. The optical strain sensors are disposedbetween the first beam and the second beam. The optical strain sensorsare placed in tension or compression as the second beam pivots withrespect to the first beam.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, which form a part of the specification andare to be read in conjunction therewith and in which like referencenumerals are used to indicate like parts in the various views:

FIG. 1 is a schematic representation of one embodiment of the opticalinclination sensor in accordance with the present invention;

FIG. 2 is a schematic representation of that optical inclination sensorin an inclined position;

FIG. 3 is a schematic representation of another embodiment of theoptical inclination sensor;

FIG. 4 is a schematic representation of yet another embodiment of theoptical inclination sensor;

FIG. 5 is a plan view of an embodiment of the optical inclination sensorin accordance with the present invention;

FIG. 6 is a view through line 6-6 of FIG. 5;

FIG. 7 is the cross-sectional view of FIG. 6 in an inclined position;and

FIG. 8 is a cross-sectional view of another embodiment of the opticalinclination sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A fiber optic inclination sensor or tilt meter in accordance with oneaspect of the present invention includes two or more optical fibers,each having a cleaved fiber end face that is placed in proximity to areflective tiltable surface. As used herein, a tiltable surface is asurface that maintains a consistent horizontal level even when, forexample, a container or housing in which the surface and the fibers aredisposed, is tilted. Suitable tiltable surfaces include spring loaded orbiased surfaces and liquid surfaces, for example the surface of mercury,water-based liquids, or oils. If mercury is used, care should be takenso that the surface tension between mercury and the walls of thecontainer do not affect the flatness of the tiltable surface. Eachoptical fiber end face is spaced from the reflective surface by apredetermined gap length.

Referring initially to FIG. 1, optical tilt or inclination sensor 10 inaccordance with an embodiment of the present invention is illustrated.Optical inclination sensor 10 includes at least one reflective surface12 and at least two separate optical fibers 14. As illustrated,reflective surface 12 is disposed within housing or frame 16, andoptical fibers 14 are fixed with respect to housing 16. Suitablematerials for housing 16 are materials that are compatible with theenvironment, including temperature and pressure, in which optical sensor10 is used. These materials include metals, polymers, sapphire, alumina,plastics and combinations thereof. Preferably, housing 16 is constructedfrom a material that will not deform in response to the environmentalconditions. Optical fibers 14 are fixed to housing 16 using methodsincluding conductive heating, arc welding, laser welding, or using FRITglass or solder glass. Alternatively, optical fibers 14 can be fixedlyattached to housing 16 using molecular, epoxy, or anodic attachmentmechanisms.

Reflective surface 12 can be any surface or the surface of any substancethat maintains its orientation with respect to a pre-determinedreference position, axis or frame-of-reference when housing 16 isrotated with respect to that pre-determined reference position, axis orframe-of-reference. Suitable reflective surfaces include mirrored glassor metallic surfaces that are magnetically biased, spring loaded orgimbaled and can be constructed from semiconductor materials or usingmicroeletromechanical (MEMS) techniques. Preferably, reflective surface12 is the surface of a liquid. Suitable liquids surfaces have an indexof refraction that is sufficient to cause the surface to act as a leasta partially reflective surface to the wavelength of light incidentthereon. In addition, the liquid should be sufficiently dense to providea surface that is smooth and stable and not overly sensitive to slightmotions and vibrations. Also, the liquid should be compatible with thematerials of housing 16 and optical fibers 14. Preferably, the liquidshould be reflective, such as mercury, or have a high index ofrefraction and hence a high reflectivity. The liquid can be water-basedor oil-based.

Suitable optical fibers 14 include single mode fibers, multimode fibers,polarization maintaining fibers, plastic fibers and coreless fibers.Preferably, optical sensor 10 does not include any remotely locatedelectronic components to measure tilt. Remotely located electroniccomponents are defined as components located at the point where tilt orangle of inclination is to be measured. Therefore, optical sensor 10 isnot adversely affected by electromagnetic effects found in subsurfaceapplications. Measurement of tilt with optical sensor 10 is accomplishedby passing light through optical fibers 14 and reflecting at least aportion of that light off reflective surface 12 and back through opticalfibers 14. Therefore, optical sensor 10 includes an interferometer. Inone embodiment, optical sensor 10 includes an extrinsic Fabry Perotinterferometer (EFPI).

As illustrated, optical fibers 14 are spaced apart by a distance 24.Each optical fiber 14 is arranged with end 18 spaced from reflectivesurface 12. As illustrated, optical fibers 14 are spaced from reflectivesurface 12 by gaps having a first gap length 20 and a second gap length22. First and second gap lengths 20, 22 have initial values that providefor a sufficient amount of tilt or movement of reflective surface 12with respect to ends 18. First and second gap lengths 20, 22 can have aninitial value (FIG. 1) that is either equal or different.

As is illustrated in FIG. 2, housing 16 and optical fibers 14 rotateabout the pre-determined reference position, for example about one ormore axes. Since optical fibers 14 are fixed to housing 16, when housing16 rotates, optical fibers 14 rotate, maintaining a consistentorientation among housing 16 and optical fibers 14. However, reflectivesurface 12 maintains its orientation with the desired reference frameand moves relative to optical fibers 14 and housing 16. As reflectivesurface 12 moves relative to optical fibers 14, first and second gaplengths 20, 22 change. This change, and in particular the differencebetween first and second gap lengths 20, 22 associated with opticalfibers 14 is used to calculate angle of inclination 26 of optical sensor10 with respect to the reference position. For example in the embodimentas illustrated, first optical fiber 28 having first gap length 20 (g₁)and second optical fiber 30 having second gap length 22 (g₂) areseparated by distance of separation 24 (L). Angle of inclination 26 (θ)is calculated using the formula tan θ=(g₂−g₁)/L. In one embodiment,distance or length of separation 24 is about 2.5 cm, and the differencebetween second gap length 22 and first gap length 20 is about 0.1 nm.The measurable angle of inclination 26 is about 0.229×10⁻⁶ degrees.

An advantage of using at least two fibers, e.g., first and secondoptical fibers 28 and 30, is the avoidance of common mode path lengthchanges due to temperature, pressure or stresses on housing 16.

The resolution of this embodiment of optical sensor 10 depends upon theability to resolve the changes or differences in first and second gapslengths 20, 22. In one embodiment, optical sensor 10 has a resolutionfor changes in gap length of less than about 1 nm. Preferably, opticalsensor 10 has a resolution for changes in gap length of about 0.1 nm,corresponding to angular resolution in the nano-radian range. Thisresolution and first and second gap lengths 20, 22 are measured usingthe interference patterns generated by the interferometer.

Various arrangements of housing 16 and optical fibers 14 can be useddepending upon the requirements and space limitations of the opticalsensor 10 application. In one embodiment as illustrated in FIG. 3,housing 16 is a generally u-shaped capillary tube 31 having first end 32in which first optical fiber 28 is disposed, and second end 34 in whichsecond optical fiber 30 is disposed. The liquid is disposed in capillarytube 31, and optical fibers 14 are fixedly attached to the respectiveends 32, 34 of capillary tube 31. Each optical fiber 14 can be sealed inrespective ends 32, 34 of capillary tube 31 so that the gaps contain apartial vacuum. In this embodiment, distance of separation 24 isequivalent to the spacing between first and second ends 32, 34 ofcapillary tube 31. In one embodiment, spacing 24 between ends ofcapillary tube 31 is about 2.5 cm, and the difference between the gaplengths 20, 22 is about 2.2 mm. The measurable angle of inclination 26is about 5 degrees.

In another embodiment as illustrated in FIG. 4, optical sensor 10includes a plurality of optical fibers 14. Each optical fiber 14 end 18is spaced from reflective surface 12. In one embodiment, all of the gaplengths are equal. In another embodiment at least two or more of the gaplengths are different. Alternatively, all of the gap lengths can bedifferent. In order to provide for attachment of plurality of fibers 14to housing 16, optical sensor 10 includes manifold assembly 36containing at least two ports 38. Preferably, a sufficient number ofports 38 are provided so that each optical fiber 14 is disposed in oneport 38. Materials for manifold assembly 36 and methods for attachingoptical fibers 14 to ports 38 are the same as discussed above forhousing 16. The measurable angle of inclination is a function of thespacing 24 between any two fibers and the gaps between said fibers andthe reflective surface.

In another embodiment as illustrated in FIGS. 5-7, optical inclinationsensor 10 includes mass 40 attached to frame or housing 42. Mass 40 ismoveable relative to housing 42. Suitable materials for mass 40 includemetal or other sufficiently dense materials so that mass 40 will movewith respect to housing 42 as mass 40 and housing 42 are rotated aboutone or more axes. Mass 40 can be arranged as any shape that fits withinhousing 42. In one embodiment, mass 40 is cylindrical. Housing 42 can beconstructed from the same material as mass 40 or from a differentmaterial. Suitable shapes for housing 42 include cup shapes and one ormore intersecting u-shaped arms. In one embodiment, housing 42 isgenerally u-shaped, having two or more upstanding arms 46 and each pairof arms 46 connected by base section 48. An optical strain sensor 44 isdisposed between mass 40 and one of the arms 46. Mass 40 is moveablyattached to base section 48. In one embodiment, mass 40 is pivotallyattached to base section 48. In another embodiment, mass 40 is attachedto base section 48 by one or more elastic tethers 50.

At least two optical strain sensors 44 are disposed between mass 40 andhousing 42 at two separate and preferably opposite locations. Suitableoptical strain sensors include, but are not limited to, an EFPI andintrinsic optical sensors such as fiber Bragg gratings. In oneembodiment, each optical strain sensor 44 is an EFPI having a gapcomprising a predetermined length, and each sensor 44 is attachedbetween mass 40 and housing 42 so that movement of mass 40 with respectto housing 42 changes the length of each gap. In general, strain sensors44 are arranged to provide for strain measurements as mass 40 moves withrespect to housing 42 when mass 40 and housing 42 are rotated about oneor more axes. As mass 40 moves with respect to housing 42, each opticalstrain sensor 44 is placed in compression or tension. The compressionand tension of optical strain sensors 44 are used to calculate angle ofinclination 26 of housing 42 and mass 40 with respect to each axis andhence angle of inclination 26 of optical sensor 10.

In one embodiment, optical strain sensors 44 are arranged in pairs andopposite to each other. At least two optical strain sensors 44 aredisposed between mass 40 and housing 42 for each axis about which sensor10 can be rotated. As illustrated, two sets of pairs of optical strainsensors 44 are provided and arranged to measure tilt about first axis 52and second axis 54. As illustrated in FIG. 7, when optical sensor 10tilts with respect to first axis 52 by angle of inclination 26, firstoptical sensor 56 is in compression when second optical sensor 58 is intension.

Although optical inclination sensor 10 provides a higher degree ofsensitivity and accuracy when applied in a stationary or static state,in one embodiment a spin or rotation is imparted to optical sensor 10.In the embodiment illustrated in FIGS. 5-7, both mass 40 and housing 42are rotated about an axis. When the axis is vertical or perpendicular tofirst axis 52 and second axis 54, the forces, i.e. tension andcompression, acting on optical strain sensors 44 are constant and equal.When the axis of rotation is tilted away from the vertical axis, theforces acting on the sensors become uneven, i.e., when the mass isrotated through the lower arc the applied force has a vertical componentparallel with gravity and when the mass is rotated through the upper arcthe applied force has a vertical component in the opposite direction asgravity. The degree of tilt is directly related to the differencebetween the highest and lowest applied force. In another embodiment, asingle mass 40 connected to a rotating spindle via an optical straingage can be used. Again, the tilt is related to the difference betweenthe highest measured strain and the lowest measured strain.

Suitable motors to rotate the mass and/or housing include a smallelectric or electromagnetic motor and servo motors. The rotational rateof mass 40 and housing 42 can be varied to vary the sensitivity ofoptical sensor 10. In other words, when the tilt angle is relativelysmall, higher rotational rate increases sensitivity of the sensor.

In another embodiment as illustrated in FIG. 8, housing 42 is arrangedas first beam 60 and mass 40 is arranged as second beam 62. Second beam62 is disposed generally orthogonal to first beam 60 and each opticalstrain sensor 44 is disposed between first beam 60 and second beam 62.Suitable shapes for first beam 60 include cylindrical or rectangularrods, and suitable shapes for second beam 62 include cylindrical orrectangular rods, and rectangular or circular disks. In one embodiment,second beam 62 is pivotally attached to first beam 60. As optical sensor10 is tilted, second beam 62 pivots with respect to first beam 60placing optical strain sensors 44 in compression or tension. Asillustrated, first optical strain sensor 56 is in compression and secondoptical strain sensor 58 is in tension when second beam 62 pivots withrespect to first beam 60 in first direction 64. Conversely, firstoptical strain sensor 56 is in tension and second optical strain sensor58 is in compression when second beam 62 pivots with respect to firstbeam 60 in second direction 66.

Optical inclination sensor 10 provides the benefit of high sensitivityin inclination measurements by using interferometry in a completelyoptical system employing fiber optic telemetry. The need for parallelelectrical systems to power and to condition the electronic outputs ofthe sensors is thereby eliminated (except when an electrical motor isneeded to rotate the sensor as described above). High sensitivity isrealized by using demodulators to resolve optical phase shifts on theorder of micro-radians. Additionally, optical path lengths of tens orhundreds of meters are incorporated in sensors of modest physicaldimensions. The combination of high interferometric demodulatorresolution and long optical path length creates the possibility ofdisplacement measurements with resolutions on the order of one part in10¹¹ to 10¹⁴.

The signals returned by sensor 10 of the present invention can beprocessed by any known techniques, including but not limited to theprocessing techniques are disclosed in U.S. Pat. Nos. 5,798,521,6,545,760 and 6,566,648, among others.

While it is apparent that the illustrative embodiments of the inventiondisclosed herein fulfill the objectives stated above, it is appreciatedthat numerous modifications and other embodiments may be devised bythose skilled in the art. Therefore, it will be understood that theappended claims are intended to cover all such modifications andembodiments, which would come within the spirit and scope of the presentinvention.

1. An optical inclination sensor comprising: a mass attached to ahousing and moveable relative to the housing as the mass and housing arerotated about at least one; and at least two optical strain sensorsdisposed at two separate locations between the mass and housing; whereinas the mass moves with respect to the housing, each one of the opticalstrain sensors are placed in compression or tension and the compressionand tension of the optical strain sensors are used to calculate an angleof inclination of the housing and mass with respect to each axis ofrotation.
 2. The optical inclination sensor of claim 1, wherein theoptical strain sensors are arranged as pairs and positioned so that oneoptical sensor is in compression when the other optical sensor is intension as the mass and housing rotate about a given axis.
 3. Theoptical inclination sensor of claim 1, wherein the housing is agenerally u-shaped housing having two arms and a base section, eachoptical strain sensor is disposed between the mass and one of the twoarms and the mass is moveably attached to the base section.
 4. Theoptical inclination sensor of claim 3, wherein the mass is pivotallyattached to the base section.
 5. The optical inclination sensor of claim3, wherein the mass is attached to the base section by one or moreelastic tethers.
 6. The optical inclination sensor of claim 1, whereineach optical strain sensor comprises an extrinsic Fabry Perotinterferometer having a gap comprising a predetermined length and eachsensor is attached between the mass and the housing so that movement ofthe mass with respect to the housing changes the length of each gap. 7.The optical inclination sensor of claim 1, wherein each optical strainsensor comprises a fiber Bragg grating.
 8. The optical inclinationsensor of claim 1, wherein the housing comprises a first beam, the masscomprises a second beam arranged generally orthogonal to the first beamand each optical strain sensor is disposed between the first beam andthe second beam.
 9. The optical inclination sensor of claim 8, whereinthe second beam is pivotally attached to the first beam.
 10. The opticalinclination sensor of claim 8, wherein each optical strain sensorcomprises an extrinsic Fabry Perot interferometer having a gapcomprising a predetermined length and each sensor is attached betweenthe first and second beams so that movement of the second beam withrespect to the first beam changes the length of each gap.
 11. Theoptical inclination sensor of claim 8, wherein each optical strainsensor comprises a fiber Bragg grating.
 12. The optical inclinationsensor of claim 1, further comprising at least two optical strainsensors disposed between the mass and housing for each axis about whichthe sensor can be rotated.